1. Field of Invention
The present invention relates to the design and construction of a mill roll used for the grinding of a juicy material such as sugar cane, and for extracting the juice therefrom.
2. Prior Art
Conventionally, a mill for grinding a juicy material such as sugar cane consists of two or more inter-meshing rolls. The most basic types are the 2-roll type which includes a top roll and a bottom roll and the 3-roll type which includes a top roll and two bottom rolls. Generally, each roll comprises a forged steel shaft shrink fitted or otherwise sleeved with a cast iron cylindrical shell. The outer peripheral surface of the shell is formed with a plurality of circumferentially extending V-shaped grooves, evenly spaced axially. Through an appropriate power drive, the top and bottom rolls are rotated in such opposing directions that sugar cane being fed into the mill between the top and bottom rolls will be crushed and the juice contained therein extracted. The bottom roll is further provided with a plurality of the so-called `messchaert grooves` along the bottom of the v-shaped grooves such that the extracted juice, flowing in these grooves, can be collected into the juice receiver underneath the mill.
An inherent disadvantage of this conventional mill is that a substantial amount of the extracted juice inevitably becomes trapped at the upper part of the cane blanket and can only reach the bottom roll by percolating through the cane blanket, so that much of the extracted juice is lost to reabsorption by the cane during the process. The accumulation of excess liquid also causes slippage of the rolls thereby adversely affecting milling capacity as well as extraction effectiveness.
An attempt to deal with these disadvantages of the conventional apparatus is disclosed in U.S. Pat. No. 3,969,802 wherein a top roll comprising a steel body with a plurality of peripheral grooves formed circumferentially thereon, has a plurality of channels within the body of the roll extending axially in the roll throughout the roll body, and a plurality of holes extending from the bottom of the grooves to communicate with corresponding channels in such a way that during operation, extracted juice can be injected radially inwardly through the holes into the channels and axially out the channels from both ends of the roll into the juice receiver. By so doing, recovery of the juice is increased. The walls of such holes are divergent in the radially inward direction and the juice channels are also divergent towards both ends of the roll.
To compensate for the inferior wearability of the steel material from corrosion and abrasion compared to cast iron, and the tendency for steel to become `shiny` with use and therefore slippery, the sides of the grooves have to be spot-welded to increase their surface roughness and to minimize wear and slippage.
U.S. Pat. No. 4,391,026 follows the same operating principle of U.S. Pat. No. 3,969,802, but defines in greater details one elaborate method of constructing the divergent openings. According to this patent, the top roll consists of a shaft, a cylindrical roll body affixed to the shaft, a plurality of circumferentially extending v-grooves formed in the periphery of the roll body, a plurality of channels extending axially through the roll body at positions inwardly of the grooves, a plurality of radially extending circular recesses formed at locations spaced circumferentially around each said groove and each extending inwardly from the bottom of the respective groove to communicate with a respective channel through another circular hole smaller in diameter than the connecting recess, and a plurality of inserts, each of which is fitted and secured within a respective recess by weld refilling the v-groove surfaces destroyed by the recess, each said insert having an elongated opening with a substantially rectangular circumferential cross-section with a longer dimension extending substantially circumferentially of the roll body and a narrower dimension extending axially of the roll body, said rectangular opening being divergent towards the center of the roll.
While these arrangements have been an improvement over conventional mill rolls, several disadvantages still exist. For example, the holes still tend to become clogged and the manner of formation of the divergent openings and their subsequent maintenance and replacement are all tediously complicated. Furthermore, because both inventions call for a substantial amount of welding, steel instead of cast iron is chosen as the material for the shell because of its easier weldability. However, its inferior resistance to wear from corrosion and abrasion has costly consequences because of the high acidity and corrosiveness of the cane juice. Much more time and expense need to be devoted to the surface welding of the roll as part of its regular maintenance.
In addition, both patents have another common disadvantage in that once any row of openings on the roll has rotated past the pinch or point of maximum compression, extracted juice remaining in the channel not yet drained off from both ends of the roll tends to flow back through the openings and be reabsorbed by the compressed bagasse. FIGS. 1A and 1B show respectively a 2-roll and a 3-roll type mill in operation. When one of the circumferentially provided juice inlet openings 2 on top roll 1 has rotated past the pinch point `x`, the juice extraction pressure becomes drastically reduced. As a result, extracted juice not yet drained off from both ends of the roll and therefore still remaining in the axially formed juice channel 3 will tend to flow back out of the roll through various juice inlet openings 2 communicating directly with the channel 3 and be reabsorbed by the outgoing cane bagasse. Similarly, as is obvious from FIG. 1B, the same disadvantage exists for the 3- roll mill, seriously affecting its extraction effectiveness.
Furthermore, because the juice inlet openings of both patents are provided at the bottom of the circumferentially provided v-shaped grooves, the width of the juice inlet opening is limited by the width of the bottom of the v-shaped grooves and the length of the juice inlet opening is limited by the diameter of the juice channel. The total cross-sectional area of the juice inlet openings cannot therefore be effectively increased.
The present invention is an attempt by the applicant to overcome or alleviate the disadvantages of the prior art described above.